Soil acidity and nutrient supply ratio as possible factors determining changes in plant species diversity in grassland and heathland communities W.J. Roem*, F. Berendse Nature Conservation and Plant Ecology Group, Department of Environmental Sciences, Wageningen Agricultural University, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands Received 4 June 1998; received in revised form 25 February 1999; accepted 9 March 1999 Abstract To elucidate the causes of the sharp decline in plant species diversity in heathland and nutrient-poor grasslands in The Nether- lands, we investigated the spatial variation in plant species richness and the abundance of threatened plant species in relation to soil acidity and soil nutrient supply ratio. We selected 68 plots divided equally between species-rich and species-poor parts of the study area, and collected data on soil characteristics, above-ground biomass and vegetation composition in each plot. In addition, we used phytometers planted with Molinia caerulea tillers to measure the nutrient supply in the soil. Soil acidity was the variable most strongly correlated with plant species diversity in heathland and grassland communities. We found that increased N:P and N:K ratios in plant biomass had additional negative eects. The Red List species in the data set and all other species that had been declining sharply in the Netherlands since 1950 were growing in soils with pH >5 and most were growing in soils with balanced nutrient supply ratios. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Biodiversity; N:P ratio; N:K ratio; Acidi®cation; Heathlands 1. Introduction Changes in land use have caused the total area of heathland and nutrient-poor grassland in The Nether- lands to diminish dramatically. In addition the species composition of the surviving areas has also changed rapidly in recent decades, with many native plant species disappearing or becoming rare (Bobbink and Willems, 1987; Diemont, 1996). These changes in species composi- tion have been mainly attributed to increased atmospheric deposition of sulphuric and nitrogenous compounds (Berendse and Aerts, 1987; Aerts and de Caluwe, 1989; Houdijk and Roelofs, 1991; Roelofs et al., 1996), which alters the chemical status of the soil by increasing nitrogen availability, inducing soil acidi®cation and mobilising toxic metal ions (Van Breemen et al., 1982; Houdijk et al., 1993; KloÈ ppel et al., 1997; De Graaf et al., 1998). Nitrogen enrichment by atmospheric deposition is also an international problem (Jeeries and Maron, 1997) causing changes in species assemblages. In order to conserve the remaining species-rich heathlands and the remaining populations of the endangered species in The Netherlands, we need to understand the mechan- isms responsible for the observed decline in species richness and in individual species. Increased nutrient availability is known to reduce species richness in grassland communities (Berendse and Elberse, 1990) by increasing the productivity of the grassland and the intensity of competition for light, thereby favouring a few tall, fast growing grasses that subsequently replace the slower growing herbs or shrubs (Grime, 1979; Berendse, 1983). In heathlands too, com- petition between grasses and heather species is changed by increased nutrient availability (Berendse and Aerts, 1987), but here most of the rare species disappeared before grasses became dominant (Houdijk et al., 1993). Species richness in herbaceous vegetation is related to nutrient availability. Grime (1979) described a hump- shaped curve of species richness versus standing crop, with few species at sites of low or high productivity, and numerous species at moderate productivity. Species 0006-3207/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(99)00049-X Biological Conservation 92 (2000) 151±161 www.elsevier.com/locate/biocon * Corresponding author. Fax: +31-317-484-845. E-mail address: wilma.roem@staf.ton.wau.nl (W.J. Roem).